1. Field of the Invention
The present invention relates to an apparatus for generating subtitle sub-picture units that compose a sub-picture stream which is interleaved with a video stream into a system stream, and especially relates to a sub-picture unit generation apparatus for subtitles that are superimposed on moving images reproduced from a video stream.
2. Description of the Background Art
In recent years, DVDs (Digital Versatile Discs) have increasingly been used as high-capacity record mediums.
A DVD records data called a system stream which is created using MPEG2 (Moving Picture Experts Group 2) video-compression techniques. A video stream, an audio stream, and a sub-picture stream are interleaved into the system stream.
The sub-picture stream includes still images which are superimposed on moving images reproduced from the video stream. When the video stream relates to a movie film, the sub-picture stream is composed of sub-picture units that correspond to subtitles used in the movie film. Each sub-picture unit includes a still image showing a subtitle, a subtitle display position, and a subtitle display start and stop time.
Conventional sub-picture units generated according to DVD authoring systems are disclosed in Japanese Laid-Open Patent Application No. 7-288776 "Sub-Picture Processing System and Storage Medium".
In this sub-picture processing system, each subtitle character string is read from a text file and rasterized in monochrome to obtain a monochrome bitmap, which is used to create a subtitle image. This subtitle image is then combined with data showing a subtitle image display position and a subtitle image display start and stop time to form a sub-picture unit.
In this processing, a display position of each subtitle image is determined by the user specifying the X and Y coordinates of the top left of a subtitle image on a display screen.
Thus, in conventional systems of generating sub-picture units for subtitles, it is necessary for the user to specify a display position of each subtitle on the screen. When a huge number of subtitles are present, such as in a movie film, it takes a considerable time to specify each subtitle display position. Also, subtitles for a movie film are usually centered on the lower part of the screen, and the coordinates of the top left point of a subtitle image display area need to be specified to determine a display position of the subtitle image. Since the size of subtitle images differs depending on the number of characters included in a subtitle, the user has to specify the X and Y coordinates of the top left point of each subtitle image on the display screen according to the width and height of a subtitle image.